The present invention relates to the field of non-contact probing of surface voltages of electrical circuits, devices and materials such as semiconductors. More specifically, the invention is related to scanning photoelectron emission microscopy as described in the Journal of Electron Spectroscopy and Related Phenomena, Vol. 52 (1990), pp. 811-819, in an article by Rotermund, et. al. It is also related to "Picosecond Photoelectron Scanning Electron Microscope for Noncontact Testing of Integrated Circuits" by P. May, et. al, published in Applied Physics Letters, Vol. 51, No. 2, of Jul. 13, 1987.
In the fabrication of semiconductor circuits it is useful to probe the electrical potential at various locations on a semiconductor sample. The most direct means for doing this is to make electrical contact at a location by means of a physical contact probe. This approach is frequently impractical because the location to be tested is not readily accessible. Contactless voltage measurement techniques offer the advantage of not requiring physical contact to the test sample. A specific application for which contactless voltage measurement is useful is in the development of photodetector arrays where it is desirable to determine whether all individual detectors are functioning after an array is delineated and prior to further processing.
One method of making non-contact voltage measurements is to use an electron-beam as a probe. U.S. Pat. No. 4,902,967, "Scanning Electron Emission by Photovoltage Contrast Imaging," describes an application of electron-beam voltage probing in conjunction with photomodulation of the sample surface voltages. This technique can be used to test photodetector arrays by observing voltage changes as the photodetectors are illuminated. However, there are limitations to the use of electron beam probing for this application. First, the electron beam generates charge carriers in semiconductor materials which can compete with the photomodulation effect that is to be observed. Second, the electron beam can induce permanent charge trapping in insulating layers on some samples which damages the sample under test. Third, the voltage resolution of the electron beam technique is reduced by the energy spread of the secondary electrons, and the resolution may be inadequate for some applications. Fourth, the electron beam can cause alterations in surface contamination layers, adversely affecting the voltage measurements. Therefore, there is a need for a non-contact method for testing the voltage characteristics of semiconductor devices that avoids the aforementioned problems associated with non-contact techniques employing an electron beam.